Within the last decade, an amount of literature has been published describing various polycrystalline, microcrystalline, or non-vitreous fibers and other shaped articles of refractory metal oxides. These articles are formed by various non-melt processes, such as by drying films of solutions of oxygen-containing metal compounds, or drying organic polymeric bodies, such as cellulose or rayon, impregnated with such a solution, or by extruding and drawing, or spinning, viscous fluids of such metal compounds into fibers. The fibers are then heated to remove water, organic material, and other volatile material to produce a refractory article. A review of the state of the art of polycrystalline inorganic fibers appears in Chapter 4 of Bracke, H. et al., "Inorganic Fibers and Composite Materials", published by Pergamon Press, N.Y., (1984). Other art in this area is Netherlands Pat. No. 7,015,245, British Pat. No. 1,287,288, U.S. Pat. Nos. 3,385,915, 3,632,709, 3,663,182 and the art cited in U.S. Pat. No. 3,709,706. Oxide fibers other than those identified as fiberglass are still in the relatively early stage of development.
Bicomponent fibers are known in the textile art. U.S. Pat. No. 2,313,296 teaches bicomponent sheath and core fibers or filaments of glass.
It is well-known in the art to prepare monocomponent ceramic fibers from spinning sols. For example, alumina-silica fibers are disclosed in U.S. Pat. No. 4,047,965; alumina-boria-silica fibers are taught in U.S. Pat. No. 3,795,524; titanium dioxide fibers are disclosed in U.S. Pat. No. 4,166,147; zirconia-silica fibers are disclosed in U.S. Pat. No. 3,709,706.